Musnad Ahmad 834:2

Waki’ narrated to us, from Abu Sufyan ibn al-‘Ala’, from al-Hasan, from Ibn Mughaffal, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the prayer time arrives and you are in the sheepfolds, then pray. And when it arrives and you are in the resting places of the camels, then do not pray, for they were created from the devils."